Drug addiction is defined as a compulsive pattern of drug-seeking and drug-taking behavior that takes places at the expense of most other activities. To address the question why addicts find it so difficult to stop using drugs, much research has been aimed at characterization ol brain systems that mediate the rewarding effects of addictive drugs. The brain reward circuits include dopaminergic innervations from ventral tegmental area and substantia nigra to the nucleus accumbens (ventral striatum) and caudate putamen (dorsal stnatum) as well as glutamate inputs from the prefrontal cortex, amygdala and hippocampus. The dendritic spines of medium spiny neurons in striatum are the cellular location for the integration of dopamine and glutamate transmission both of which are important for development and expression of the adaptive effects of psychostimulants. Because of the long lasting aspects of drug addiction, reorganization of synaptic connections and their maintenance have been suggested as a cellular mechanism of learning and enduring memory associated with addictive behaviors. However, the function and molecular mechanism of psychostimulant-induced dendritic spine proliferation are not fully understood. We propose to investigate cocaine-induced spine proliferation and its physiological significance in two major neuronal subpopulations in striatum: striatonigral and striatopallidal neurons (Aim I). We hypothesize that reorganization of actin filaments may underlie the mechanisms involved in spine proliferation induced by psychostimulants. Regulators of the actin cytoskeleton may play an essential role in drug-induced spine formation. We therefore propose to study five regulators of actin dynamics as targets of psychostimulants in spine proliferation: Cdk5 (Aim II),WAVE1 (Aim III)as a new substrate of Cdk5, spmophilin (Aim IV), neurabin (Aim IV) and Lfc (Aim IV) as a new molecule interacting with spinpphilin/neurabin. hi order to address the role of the five key regulators in psychostimulants-induced spine proliferation, we will analyze the biochemical, cell morphological, electrophysiological and behavioral responses to administration of cocaine in neuronal type-specific or conventional knockout mice and wild type mice. Furthermore, to identify novel targets involved in the actions of psychostimulants on spine formation, we propose to perform quantitative proteomic analysis of purified post-synaptic density after chronic treatment with cocaine (Aim I). Taken together, these studies will lead to elucidation of molecular mechanism(s) leading to spine formation after repeated exposure to addictive drugs and the role of spine formation in the actions of psychostimulants.